Optical sensor for detecting moisture on a surface

ABSTRACT

An optical sensor ( 5 ) for recording wetting of a surface ( 20 ) of a member ( 21 ), especially a window of a motor vehicle, having a transmitter ( 13 ), a receiver ( 14 ) and a retroreflector ( 11, 11′, 11″ ) for electromagnetic waves is proposed. The surface ( 20 ) of the member ( 21 ) has for this at least two sensing regions ( 12, 12 ′), which are exposed to electromagnetic waves originating from the transmitter ( 13 ) and coupled into the member ( 21 ) in a central region ( 10 ), the development of wetting on the sensing regions ( 12, 12′ ) effecting a change of the signal of the electromagnetic waves coupled out into the receiver ( 14 ) in the central region ( 10 ). 
     For this, the retroreflector ( 11, 11′, 11″ ) is developed and positioned in such a way that it feeds back electromagnetic waves reflected by the surface ( 20 ) of the member ( 21 ) to the surface ( 20 ) of the member ( 21 ), and from there into the central region ( 10 ).

BACKGROUND INFORMATION

An optical sensor for recording the wetting of a surface, particularlythe wetting of a motor vehicle window, is described in German PublishedPatent Application No. 199 43 887 having a transmitter and a receiverfor electromagnetic waves, the surface being present in a sensing regionbetween the transmitter and the receiver, so that the formation of awetting on the surface effects a change in the signal detected by thereceiver. Furthermore, in that location the optical sensor has a lightguiding element by which the electromagnetic waves are bidirectionallyguided into the sensing region or guided away from the sensing region.Finally, it is known from German Published Patent Application No. 199 43887 that one may position a retroreflector, particularly a holographicretroreflector, in such a way that it feeds back electromagnetic wavesreflected by the surface to the surface, and from there to the lightguiding element. The optical sensor thus described may be usedespecially as a rain sensor, the light coupled in when the window is drybeing totally reflected at the window's outer surface, whereas, when thewindow is wet, the total reflection is interrupted, which is whateffects the detected signal change.

Another rain sensor is described in European Published PatentApplication No. 0 999 104 in which a holographic film is situatedbetween sensor and receiver.

SUMMARY OF THE INVENTION

As compared to the related art, the optical sensor according to thepresent invention has the advantage that it has a larger sensitivesurface. This makes it possible to increase the sensitivity of thesensor and to lower the response threshold, while at the same timeminiaturizing the evaluation electronics and the other electroniccomponents, such as the transmitter and the receiver.

Besides that, the optical sensor according to the present invention alsomakes it possible to classify the intensity of the rain, so that thesensor signal is not only able to be used for switching on or off awindshield wiper of a motor vehicle, but also, for example, forcontrolling the wiper's frequency or rather the length of the intervalsbetween individual wiping procedures.

A further advantage of the optical sensor according to the presentinvention is that, because of the plurality of sensing regions provided,it is able to keep functioning even when one of these sensing regionsfails, i.e. it is overall more reliable and less susceptible to faults,or rather has a redundant function in this respect.

Finally, because of the plurality of sensing regions provided, anoverall more favorable arrangement of the retroreflectors relative tothe central region is achieved, or rather to the electronic componentsthat are in connection with the central region, which leads to anoverall better usage of the available surface of the field, which may,in the case of motor vehicle windows, be as small as possible forreasons of an undesired impairment of the view of the driver.

Thus, it is advantageous, with respect to a miniaturization that goes asfar as possible, if the at least one transmitter and the at least onereceiver are present in the central region.

Besides that, the central region may advantageously be made smaller,with respect to the surface required, in that the transmitter and/or thereceiver are accommodated in a separate component outside the centralregion. In this case then, as already described in German PublishedPatent Application No. 199 43 887, a first light guiding element that isin contact with the transmitter, and a second light guiding element thatis in contact with the receiver, are guided from this component into thecentral region, and they are additionally connected to a coupling-inelement or coupling-out element that are situated in the central regionand are respectively associated with the appropriate light guidingelement.

In order to achieve a surface usage that is as optimal as possible and acompact type of construction, it is also advantageous if an even number,particularly two, four or six sensor regions are provided which lieopposite to each other, pair by pair, with respect to the centralregion.

When it comes to the aspect of the coupled-in electromagnetic wavesbeing applied to the sensing regions in as simple, as uniform and ascomplete a manner as possible, as well as of as good as possible asurface usage, it is additionally advantageous if the sensing regionsare developed fan-shaped, circular segment-shaped, circularsector-shaped or circular ring-shaped. By the way, however, instead ofthe plane shapes related to a circle, plane shapes related to an ellipsealso come into consideration.

The retroreflector is advantageously a holographic retroreflector whichhas two focal points, in one of the focal points the transmitter and inthe other the receiver being situated, or rather, in one of the focalpoints the coupling-in element for the sensor and in the other focalpoint the coupling-out element for the receiver being located.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a section through a window of a motor vehicle which isprovided with an optical sensor in the form of a rain sensor.

FIG. 2 shows a top view on the lefthand side or the righthand side ofFIG. 1 along the section line drawn in there.

FIG. 3 shows a top view of a further exemplary embodiment similar toFIG. 1, for an optical sensor in the form of a rain sensor.

FIG. 4 shows the optical path in a holographic retroreflector having twofocal points.

DETAILED DESCRIPTION

FIG. 1 shows a section through a window 21 of a motor vehicle, on whichon one side there is a central region 10, from which, as seen in the topview, two semicircular electromagnetic waves proceed, which impinge upontwo semicircularly designed sensing regions 12 associated with them,from there are reflected in each case, or rather are totally reflected,provided window 21 is dry on its surface 20 in the area of sensingregions 12, from sensing regions 12 strike two semicircularretroreflectors 11 associated with sensing regions 12, which aredesigned in the form of holographic retroreflectors 11 having two focalpoints 22, 23, and are then fed back again by the retroreflectors 11 viasensing regions 12 into central region 10.

FIG. 2 shows a top view of FIG. 1, only one half of central region 10,or rather one of the two semicircular sensing regions 12 and theassociated semicircular retroreflectors 11 being shown.

Thus, overall, central region 10, together with sensing regions 12,retroreflectors 11 and window 21 which has the electromagnetic wavesimpinging on it, forms an optical sensor 5 which is sensitive to wettingof sensing regions 12, and thus of window 21.

In particular, in central region 10 a change in the intensity of theelectromagnetic waves reflected back to there from the two sensingregions 12 is detected separately for the two, as soon as, for example,in at least one of sensor regions 12, by the formation of wetting byrain or fog, the condition for the appearance of total reflection of themagnetic waves irradiating sensor regions 12 changes, i.e. the totalreflection given when window 21 is dry is interrupted when window 21 iswet.

In the explained exemplary embodiment as in FIG. 1, central region 10also has two coupling-in elements corresponding to the number of sensorregions 12, via which sensor regions 12 are able to be impinged uponwith electromagnetic waves by two light-emitting diodes integrated intocentral region 10, which are each provided as transmitters 13.

However, instead of two light-emitting diodes in central region 10, onemay also provide only one light-emitting diode, as common transmitter 13for all sensor regions 12, downstream from which is placed a customarybeam splitter which is adapted to the number of sensing regions 12. Bythe way, the beam splitter may also be integrated into the coupling-inelement.

Besides that, central region 10 according to FIG. 1 or 2 also has twocoupling-out elements per number of sensing regions 12, via which thechange in the measuring signals occurring in sensing regions 12 may bedetected using two separate receivers 14, such as photodiodes, which areeach assigned to the coupling-out elements and integrated into thecentral region.

Preferably, a separate receiver 14 is provided in central region 10 foreach sensing region 12, so that a spatially resolved change in thesignal is possible, i.e. an assignment of a signal change to a sensingregion 12.

However, transmitter 13 and/or receiver 14 may also be positionedoutside central region 10, as an alternative to positioning them incentral region 10, as is known from DE 199 43 887 A1. In this case,then, transmitter 13 and/or receiver 14 are connected to the coupling-inelement and coupling-out element, associated in each case and situatedin central region 10, via a light guiding element, such as a monomode ormultimode light guide or a bundle of such light guides.

FIG. 3 shows a further exemplary embodiment of an optical sensor 5having three sensing regions 12, which each cover an angular region ofca 90°, the three retroreflectors 11, assigned to the three sensingregions 12 being also structured in a corresponding circular ring shape.The exemplary embodiment according to FIG. 3 is largely analogous to theexemplary embodiment according to FIGS. 1 and 2, apart from the numberand shape of sensing region 12′ and retroreflectors 11′ as well as thecorrespondingly adapted design of central region 10.

A continuation of the exemplary embodiment according to FIG. 3 providesfor forming sensing regions 12′ in such a way that they cover an angularregion of 30° to 120° in each case, especially 60° to 120°. In thiscontext, the number of circular ring-shaped sectors or sensing regions12′ may also be other than three, in deviation from FIG. 3. Preferablythe number of circular ring-shaped sectors or sensing regions 12′ liesbetween two and six, they also being formed in such a way that theycover the greatest angular range possible, ideally 360°.

In the previously explained exemplary embodiments it is finallypreferably provided that the shape of retroreflectors 11, 11′ is adaptedto the shape of sensing regions 12, 12′. In this connection, it isespecially preferred if both the shape and the number of retroreflectors11, 11′ correspond to the shape and number of sensing regions 12, 12′.

By the way, retroreflectors 11, 11′ according to FIGS. 1 to 3, arepreferably holographic retroreflectors 11, 11′, as is known from DE 19943 867 A1, which are pasted onto window 21 in the form of a foil, asheet or a flat glass plate. However, as an alternative, suchholographic retroreflectors 11, 11′ may also have been integrated intothe interior or the surface of window 21, for instance, as early asduring its production.

The coupling-out elements and the coupling-in elements which are locatedin central region 10 are formed in a known way, for example by adeflector, a prism or a grating.

Finally, FIG. 4 explains schematically the positioning of transmitter 13and receiver 14 in central region 10, in a top view, a retroreflector11″ being drawn in which is designed in the shape of a section of anelliptical ring. This retroreflector 11″ has two focal points 22, 23, sothat electromagnetic waves originating from transmitter 13 in firstfocal point 22 is reflected via sensing region 12, 12′ (not shown) andthe associated retroreflector 11″ to second focal point 23, and thusback to the location of receiver 14.

By the way, with regard to further details known per se of the design ofoptical sensor 5, we refer the reader to Application DE 199 43 887 A1.In particular, retroreflectors 11, 11′, 11″, as described there, mayalso be designed as mirror segments or concentrating reflector segments,which focus the electromagnetic waves onto the coupling-out element orreceiver 14.

1. An optical sensor for recording a wetting of a surface of a member,comprising: at least one transmitter; at least one receiver; at leastone retroreflector for electromagnetic radiation; at least two sensingregions included on the surface and being exposable to anelectromagnetic wave from the at least one transmitter; and a centralregion arranged in the member and into which are coupled the at leasttwo sensing regions, wherein: a development of the wetting on the atleast two sensing regions effects a change of a signal of theelectromagnetic wave coupled out into the at least one receiver in thecentral region, the at least one retroreflector is developed andpositioned in such a way that the at least one retroreflector feeds backthe electromagnetic wave reflected by the surface of the member to thesurface of the member, and from the surface of the member into thecentral region, and the at least two sensing regions are impinged uponby the electromagnetic wave via the central region.
 2. The opticalsensor as recited in claim 1, wherein: the surface includes a window ofa motor vehicle.
 3. The optical sensor as recited in claim 1, wherein:at least one of the at least one transmitter and the at least onereceiver is situated in the central region.
 4. The optical sensor asrecited in claim 1, wherein the at least two sensing regions have ashape that is one of: fan-shaped, circular segment-shaped, circularsector-shaped, circular ring-shaped, and a sheetlike shape using anellipse as a point of departure.
 5. The optical sensor as recited inclaim 1, wherein: the at least two sensing regions cover an angularrange of 300 to
 1800. 6. The optical sensor as recited in claim 1,wherein: the at least two sensing regions cover an angular range of 600to
 1200. 7. The optical sensor as recited in claim 1, wherein: a shapeof the at least one retroreflector is adapted to a shape of the at leasttwo sensing regions.
 8. The optical sensor as recited in claim 1,wherein: a shape and a number of the at least one retroreflector areadapted to a shape and a number of the at least two sensing regions. 9.The optical sensor as recited in claim 1, wherein: the at least onetransmitter includes separate transmitters separately assigned to eachof the at least two sensing regions.
 10. The optical sensor as recitedin claim 1, wherein: the at least one transmitter includes a commontransmitter assigned to the at least two sensing regions, and the commontransmitter includes a beam splitter downstream that is integrated intoa coupling-in element and is designed corresponding to a number of theat least two sensing regions.
 11. The optical sensor as recited in claim1, wherein: the at least one receiver includes separate receiversseparately assigned to each of the at least two sensing regions.
 12. Theoptical sensor as recited in claim 1, wherein: the at least one receiverincludes a common receiver assigned to the at least two sensing regions.13. The optical sensor as recited in claim 1, wherein: the at least oneretroreflector includes a holographic retroreflector.
 14. The opticalsensor as recited in claim 1, wherein: the at least one retroreflectorincludes two focal points, one of the focal points copying theelectromagnetic wave onto one of a coupling-in element and the at leastone transmitter, and another one of the focal points copying theelectromagnetic wave onto one of a coupling-out element and the at leastone receiver.
 15. The optical sensor as recited in claim 1, wherein: anumber of the at least two sensing regions is even, and the at least twosensing regions lie opposite each other in pairs with respect to thecentral region.
 16. The optical sensor as recited in claim 15, wherein:the number of the at least two sensing regions includes one of two,four, and six sensing regions.
 17. The optical sensor as recited inclaim 1, wherein: the central region includes at least one coupling-inelement via which the at least two sensing regions are able to beimpinged upon by the electromagnetic wave originating from the at leastone transmitter.
 18. The optical sensor as recited in claim 17, wherein:the central region includes at least one coupling-out element via whichthe change in the signal occurring in the at least two sensing regionsis able to be coupled out into the at least one receiver.
 19. Theoptical sensor as recited in claim 18, wherein: the at least onetransmitter is connected to the coupling-in element, and the at leastone receiver is connected to the coupling-out element.
 20. The opticalsensor as recited in claim 18, further comprising: a light-guidingelement, wherein: the at least one transmitter is connected to thecoupling-in element via the light guiding element, and the at least onereceiver is connected to the coupling-out element via the light guidingelement.
 21. The optical sensor as recited in claim 20, wherein: thelight guiding element includes one of a monomode light guide, amultimode light guide, a bundle of one of monomode light guides andmultimode light guides, and a plate.